Stablilization of Clay with Fly Ash and GGBS Based Geopolymer: Influence on Strength, Consolidation, and Shrinkage Characteristics

The ground beneath our feet is not always a solid, unyielding foundation. In many parts of the world, the soil is dominated by clay, a fine-grained material that behaves like a sponge. When rain falls or the seasons change, this clay absorbs water and swells, pushing upward with significant force. When the air is dry, it shrinks and cracks, pulling away from whatever rests upon it. This constant cycle of expansion and contraction is a nightmare for engineers and homeowners alike, causing roads to buckle, foundations to crack, and structures to settle unevenly. To make this soil safe for building, engineers traditionally mix in ordinary cement or lime. These binders harden and lock the soil particles together, creating a stable base. However, the production of these traditional binders is a major source of carbon dioxide emissions, contributing significantly to the global climate crisis. This has driven a search for alternatives that can stabilize the earth without the heavy environmental cost.

Researchers have turned their attention to industrial waste products, specifically fly ash and ground granulated blast-furnace slag. Fly ash is the fine powder left over after coal is burned in power plants, while slag is a by-product of making iron. Both materials are rich in the chemical ingredients needed to create a strong, rock-like substance when mixed with a specific alkaline solution. This process, known as geopolymerization, turns these waste materials into a binder that can replace traditional cement. A team of researchers at the Indian Institute of Technology Madras and Thiagarajar College of Engineering set out to test how well these industrial by-products could tame a specific type of problematic clay found in Madurai, India. Their goal was not just to see if the soil became stronger, but to understand how the treatment changed the way the soil settles under weight and how much it shrinks when it dries.

The team selected a sample of clay from a depth of two meters near a local bus stand. This soil was classified as having intermediate compressibility, meaning it squishes easily under pressure, and it was moderately expansive, with a high tendency to swell when wet. To treat this soil, the researchers mixed it with varying amounts of fly ash and slag, creating three different concentrations for each material: 25%, 35%, and 45% of the total dry weight. They activated these mixtures with a solution of sodium hydroxide and sodium silicate, which acts as the chemical trigger to start the hardening process. They also added a small amount of a chemical retarder to the slag mixes to prevent them from setting too quickly. The treated soil was then compacted into cylinders and left to cure for different periods, ranging from zero days up to 28 days, before being put to the test.

The results revealed a clear story of transformation. The untreated clay had a very low strength, measuring just 3.63 kilograms per square centimeter. When the researchers tested the soil treated with fly ash, they found that strength increased as the soil cured over time. The best performance came from the mix containing 35% fly ash, which reached a strength of 17.19 kilograms per square centimeter after 28 days. This represented a massive improvement, making the soil nearly four times stronger than it was in its natural state. However, when they increased the fly ash content to 45%, the strength actually dropped, suggesting that too much of the binder diluted the soil structure rather than strengthening it.

The story was slightly different with the slag-based mixtures. For the slag, the strength continued to climb as they added more of the material. The highest strength recorded in the entire study came from the mix with 45% slag, which reached 18.84 kilograms per square centimeter after 28 days. This was a 419% increase over the untreated clay. Because the strength was still rising at the highest amount tested, the researchers noted that the ideal amount of slag might be even higher than 45%, whereas the fly ash had already passed its sweet spot at 35%.

Beyond just making the soil harder, the treatment dramatically changed how the soil behaved under pressure. When heavy loads are placed on soft clay, the soil compresses and water is squeezed out, causing the ground to sink over time. The researchers measured this behavior and found that the treated soil was far less prone to settling. The mix with 35% fly ash reduced the soil's tendency to compress by about 70%, while the 45% slag mix reduced it by roughly 84%. Furthermore, the treated soil settled faster than the untreated clay, meaning that any remaining sinking would happen more quickly and be less of a long-term worry for structures built on top.

Perhaps the most critical finding for regions prone to drought and heavy rains was the change in shrinkage. Untreated clay shrinks significantly as it dries, creating deep cracks that can damage foundations. The geopolymer treatment significantly curbed this behavior. The mix with 35% fly ash reduced the volume of shrinkage by nearly half, while the 45% slag mix reduced it by about 63%. This means that the treated soil would remain much more stable during dry seasons, resisting the cracking and swelling that typically plague buildings on clay soils. The researchers also observed that the soil became less plastic, meaning it was less likely to deform permanently when wet, further enhancing its stability.

These findings suggest that using industrial by-products like fly ash and slag offers a powerful, lower-carbon alternative to traditional cement for stabilizing difficult soils. The study identified that 35% fly ash and 45% slag were the most effective proportions within the ranges they tested, delivering strength gains of nearly five times the original soil while drastically reducing the risks of swelling and shrinking. By turning waste materials into a high-performance binder, this approach not only solves a geotechnical problem but also addresses an environmental one, offering a way to build on unstable ground without the heavy carbon footprint of conventional cement. The work confirms that these geopolymer binders can effectively transform problematic clay into a reliable foundation material, ready to support the infrastructure of the future.